1. Technical Field of the Invention
This invention relates to heat exchange systems such as condensing and evaporating systems and, in particular, to systems wherein a volatile working fluid and its vapors are employed to effect operations on an object and the losses of the fluid and its vapor are controlled to reduce operating costs and environmental contamination. Even more specifically, the subject process and apparatus may be employed advantageously to control working fluid losses and vapor emissions in such commonplace industrial technologies as vapor degreasing, volatile solvent cleaning, condensation heating for reflow solder assembly, condensation heating for fusing solder plating, and numerous other related applications.
2. Description of the Prior Art
Over the years various processes have been developed in which an object is exposed to a volatile working fluid. For example, vapor degreasing processes and solvent cleaning processes have been employed to remove various contaminants or residues from a wide range of manufactured pieceparts. More recently, fluorocarbons have been used at elevated temperatures to effect solder fusing or solder reflow assembly on numerous electronic components.
These processes typically suffer significant losses of the volatile working fluid. Such losses affect the costs involved in the utilization of the processes. Furthermore, with increasing governmental regulations affecting health, safety and the environment, emissions of the working fluid vapors or its products of decomposition must be more stringently controlled. Existing machines and apparatus for implementing these processes are becoming increasingly inadequate.
An early example of a process utilizing a volatile working fluid is set forth in M. J. Borushko, U.S. Pat. No. 2,515,489 issued July 18, 1950. Borushko relates to a coating process adapted for use in applying various types of coatings to various types of articles. This process is adapted for applying coatings which are characterized as consisting of one or more suitable film forming components dissolved in one or more volatile solvents.
Implementation of the process involves the use of a dip tank consisting of two elements. The first element is comprised of a lower tank section which holds a solution of the coating material to be applied. This lower tank section is fitted with some heating means. The second element is comprised of an upper tank section which functions as a drying chamber and solvent vapor condenser. This upper tank section is in reality an upward extension of the sides of the lower chamber. The upper section is surrounded by a jacket through which a cooling fluid is circulated.
In practice the solution of the coating material is placed in the lower chamber. Heat is applied and when the solution has attained a suitable temperature, which may generally be said to be a temperature near the boiling point of the solvent, the article to be coated is immersed in it and held there until the article itself has become heated. It is then raised and held in the upper tank section, whereupon the heat contained within the article causes the solvents to be driven out of the film, and the solvent vapors pass to the cold sides of the chamber where they are condensed and from which they flow back into the chamber below.
While Borushko recognizes the advantage to be derived in heating the article to a temperature near the boiling point of the solvent, he does not consider the losses of solvent which may occur due to the convective interchange between the atmosphere within the apparatus and that exterior thereto. The atmosphere within the upper tank section is comprised of an air and solvent vapor mixture in which the vapor portion is continually being replenished by the evaporation of solvent from the surface of the hot coating liquid in the lower tank section as well as by the solvent vapor being driven off the article. Some of the solvent contained in the air and vapor mixture is removed by condensation onto the cold sidewalls.
The condensation process is somewhat enhanced by the natural convection of the atmosphere within the upper chamber. This natural convection is induced by the heating effects of the hot article and the hot coating liquid at the bottom of the chamber and by the chilling effect of the cold sidewalls. However, the condensation removal process cannot progress beyond the saturation level of the air and vapor mixture corresponding to the temperature of the chilled sidewalls. In practice, the partial pressure of the vapors within the atmosphere of the chamber will be well above this chilled sidewall saturation level. This results from the relatively slow convection within the chamber and the resultant necessity of the vapor diffusing over a relatively large boundary layer in order to be condensed at the chilled sidewalls. Consequently, the atmosphere within the upper chamber always contains a significant amount of solvent vapor.
In the absence of a physical closure, the natural convective forces within the upper chamber will lead to substantial convective interchange between the air and vapor atmosphere within the chamber and the environment exterior thereto. This convective interchange results in losses of the solvent and may lead to the emission of gaseous products of decomposition of the solvent as well.
Another example of a process and apparatus utilizing a volatile working fluid for cleaning rigid objects is disclosed in G. Edhofer et al, U.S. Pat. No. 3,028,267 issued Apr. 3, 1962. This patent relates to cleaning rigid objects which have become contaminated with grease, oil, dirt, metal shavings and the like. The objects to be cleaned are first degreased by the vapors of a solvent containing fillers. Then, while the objects are still moist with the solvent, they are immersed in a liquid solvent containing fillers. Subsequently, the objects are immersed in washing chambers containing liquid solvent without fillers. The major quantity of dirt and grease removed by the condensate dripping off the treated object is deflected away from the chamber containing the fillers by a drip shield or pan and the soiled condensate conducted away for distillation.
Although Edhofer et al utilize a partial draining of the objects in their process, they do so in order to capture the contaminated solvent and prevent it from being deposited back in the chamber containing clean solvent with fillers. Moreover, since the apparatus is open at the top and since the atmosphere above the solvent chambers is not controlled to limit the amount of solvent vaporized therein, significant losses of solvent occur.
Still another example of the developing technology in the use of volatile working fluids is evidenced by B. Rand, U.S. Pat. No. 3,078,701 issued Feb. 26, 1963. Rand '701 discloses an air recirculation system for cleaning apparatus. A closed circuit air recirculation system is used for withdrawing a mixture of air and vaporized solvent from within the cleaning apparatus with the air being returned to the apparatus after the concentration of the vaporized solvent is reduced.
Although in Rand '701 some control is exerted over the amount of solvent contained within the internal atmosphere, it should be noted that this control is not very effective. The reason for this relative ineffectiveness is due to the direct exposure of the internal atmosphere to the source of solvent. Consequently, while on the one hand the concentration of the vaporized solvent is reduced through the external loop, on the other hand the presence of the heated solvent in the internal atmosphere causes the concentration to be increased. Under these conditions the relative partial pressure of the vaporized solvent in the internal atmosphere cannot be effectively controlled to a sufficiently low level. Coupling this relatively ineffective control with the fact that in Rand's apparatus there is at least one opening to the external environment, the emission of solvent from the apparatus is still significant.
A further example of a process utilizing a volatile working fluid is set forth in R. C. Pfahl, Jr. et al, U.S. Pat. No. 3,866,307 issued Feb. 18, 1975. Pfahl, Jr. et al disclose that an article to be soldered, fused or brazed is placed in hot saturated vapors generated by continuously boiling heat transfer liquid having selected properties including a boiling point at least equal to, and preferably above, the temperature required for operation. Vapors condense on the article and give up their latent heat of vaporization to heat the article to the temperature needed for soldering, fusing or brazing.
While the Pfahl, Jr. et al process represents a dramatic improvement in the soldering, fusing and brazing art, the losses of the expensive working fluid increase the operating costs significantly. One approach advanced to reduce these operating costs is disclosed in T. Y. Chu et al, U.S. Pat. No. 3,904,102 issued Sept. 9, 1975. In Chu et al a blanket of secondary vapor, having a density intermediate that of the primary vapor and the atmosphere, is floated on the body of primary vapor in order to reduce the losses of the primary vapor. The article on which the soldering, fusing or brazing operation is to be performed is passed through the body of secondary vapor into the body of primary vapor in the vessel. Primary vapor condenses on the article and the latent heat of vaporization of the condensing primary vapor heats the article to the temperature required for the soldering, fusing or brazing operation. After completion of the operation, the article is withdrawn from the body of primary vapor through the blanketing body of secondary vapor, out of the vessel and into the atmosphere where it is cooled to ambient temperature.
It should be evident that in Pfahl, Jr. et al and Chu et al, the processes disclosed utilize essentially an open apparatus. As such, these processes suffer many of the same shortcomings as heretofore discussed with respect to Borushko.
A more recent illustration of the developments being made in the use of volatile working fluids is provided by B. Rand, U.S. Pat. No. 4,012,847 issued Mar. 22, 1977. Rand '847 relates to a solvent recovery system for use with a process chamber having unsealed inlet and outlet ports. Included in the system are means for recovering vaporized solvent from the chamber in two stages. The first stage comprises a chiller for removing a portion of the vaporized solvent. The second stage is comprised of an adsorber for recovery of the vaporized solvent. Additionally, means for causing the air and vaporized solvent to flow from the housing to the chiller and then to the air circulation system are also utilized.
The Rand '847 system does not address the problems involved with controlling losses of the solvent to the atmosphere. This is clearly the case since Rand specifically indicates that seals at the inlet and outlet ports are not deemed necessary. Moreover, the Rand '847 apparatus cannot be used to expose an article to a volatile working fluid to effect an operation thereon. Its use is only for recovery of any working fluid remaining on the article after its exposure to the working fluid.
One attempt to reduce working fluid losses through the use of a nominally closed chamber is disclosed in B. Rand, U.S. Pat. No. 4,029,517 issued June 14, 1977. Rand '517 relates to a vapor degreasing system having a divider wall between upper and lower vapor zone portions. The workpiece is introduced into an upper chamber and a sliding door is closed. A lower door is opened allowing vapors from a lower chamber to rise into the upper chamber. After a period of time a condenser in the lower chamber is reactivated causing the vapor level to drop. At this point the lower door is closed separating the upper chamber from the lower chamber.
While Rand '517 partially addresses the problem of working fluid losses and emissions, during the time period when the vapor is rising, it mixes with the air in the upper chamber forming an air and vapor mixture. This mixture must be displaced by the rising vapor resulting in the emission of a portion of the air and vapor mixture through the sliding door. Correspondingly, when the vapor is lowered, air must enter the upper chamber to displace the dropping vapor. As a result, it is clear that the sliding doors cannot be sealed. This lack of a tight seal allows for further fluid losses while the vapor resides in the upper chamber.
A more recent example of the advances made in systems using volatile working fluids is evidenced by D. J. Spigarelli, U.S. Pat. No. 4,077,467 issued Mar. 7, 1978. Spigarelli relates to a method and apparatus for soldering, fusing and brazing. The apparatus is such that an article to be soldered is sequentially inserted into first and second confined regions of hot saturated vapors and sequentially removed therefrom. The article remains temporarily in the first confined region of hot saturated vapors of a primary high temperature liquid such that the vapors condense on the article to heat it.
The article is then removed to the second confined region which contains a body of hot saturated vapors of a secondary liquid having a lower boiling point and density than the primary liquid. This secondary liquid serves as a vapor blanket and causes the primary vapors to condense and return to the primary liquid reservoir. The temperature of the secondary liquid is controlled independently of the primary liquid. Any primary and secondary liquids and flux remaining on the article after soldering can be removed by spraying the article with distilled secondary liquid as it passes through the secondary vapors on the removal of the article from the apparatus.
In his preferred embodiment, Spigarelli has focused on the minimization of the height and consequent cost of the apparatus, as well as the conservation of the primary working fluid. However, the apparatus is subject to significant losses of the secondary working fluid as well as some losses of the primary fluid. This result obtains because the atmosphere above the saturated secondary vapor zone, forming the vapor blanket in the second confined region, is a mixture of primary vapor, secondary vapor, and air. This atmosphere is particularly rich in secondary vapor because of diffusion and the natural convective interchange with the hot saturated vapors forming its lower boundary. It also contains primary vapors which have escaped from the first confined region and which have evaporated from the article as it is removed from the first confined region. In practice, the partial pressure of the primary working fluid is in excess of its saturation vapor pressure corresponding to the temperature of the condensers in the second confined region. Also, the partial pressure of the secondary working fluid can be expected to be well in excess of its corresponding saturation vapor pressure.
When the door of the second confined region is opened to withdraw the article, the natural convective forces within this chamber as well as the movement of the article causes significant convective interchange between the atmosphere within the second confined region and the environment exterior thereto. Consequently, the vapors contained within the atmosphere of the second confined region are subject to losses as recognized by Spigarelli. In addition, any gaseous products of decomposition of the primary and secondary working fluids which might be contained within this atmosphere also would be emitted into the environment.
A second loss mechanism exists for the Spigarelli apparatus. This loss mechanism is similar to that which exists in Rand '517. When an article is first immersed in a zone of saturated working fluid vapor having a temperature higher than the article, the vapors rapidly condense onto the workpiece, thereby causing a sudden drop in the level of the saturated vapor into which the article is immersed. This drop in vapor level will cause a reduction in the atmospheric pressure within the apparatus so that there will be a tendency for air from the exterior environment to enter the apparatus through any door or closure which is not completely sealed until the interior pressure is equilibrated with the exterior pressure. Subsequently, as the vapor is replenished and the vapor level rises, the atmospheric pressure within the apparatus will tend to rise so that there is a tendency for the vapor-laden atmosphere within the apparatus to be expelled, hence giving rise to a loss of working fluid. This problem with the "breathing" of the apparatus during the processing of an article is characteristic of other embodiments of Spigarelli as well.
A third loss mechanism for liquid working fluid exists in the preferred embodiment of Spigarelli as well as other embodiments in which liquid secondary working fluid is sprayed onto the article for the purpose of rinsing off condensed primary working fluid or for the removal of flux residue after soldering, fusing or brazing. Spigarelli does not provide for the effective draining of working fluid from the article nor does he provide for the utilization of the heat which may be provided to the article by the working fluids to enhance the later evaporative removal of the working fluids from the article. Consequently, in the absence of an active drying process performed upon the article, the removal of the article from the apparatus may also involve the loss of liquid working fluid retained upon the article.
An alternative embodiment includes the addition of a further chamber which is utilized for preheating and drying purposes. The article is initially placed within the preheating chamber. The article and any soldering flux associated therewith are heated to a temperature in excess of the boiling point of the secondary vapors in the intermediate secondary vapor chamber. This preheating permits the article and the flux material thereon to be passed through the intermediate vapor blanket on its way into the high temperature primary chamber without removal of the flux.
In utilizing this process the article tends to be cooled as it is removed through the secondary vapor blanket after the soldering, fusing or brazing operation in the primary chamber. Before removal, the article is held in the preheating chamber. It is specifically noted by Spigarelli that heaters in this chamber heat the surface of the article for drying and that condensed secondary vapors present on the article are vaporized. Such vapors then pass upwardly to cooling coils lining the upper interior walls of the preheating chamber. At the coils, these vapors are recondensed and return to the lower portion of the secondary vapor chamber.
This alternate embodiment of Spigarelli also suffers losses of the working fluid vapors, although the rate of loss is lower than for the preferred embodiment. The atmosphere within the drying chamber contains a mixture of primary working fluid vapor, secondary working fluid vapor and air as was the case in the earlier discussion. By isolating this chamber from the intermediate secondary vapor chamber, a major source of replenishment of secondary vapors within the drying chamber is eliminated. However, an initial charge of working fluid vapor is provided by convective interchange when the atmosphere of the drying chamber and the intermediate chamber are in communication during the removal of the article from the intermediate chamber into the drying chamber. In addition, the evaporation of secondary working fluid from the article upon being heated in the drying chamber provides an additional source of vapors in the atmosphere of the drying chamber.
One problem with this scheme is that the apparatus cannot provide for the effective recovery of the vapors contained within the air and vapor mixture constituting the atmosphere of the drying chamber. The utilization of a recirculation fan for the purpose of enhancing the heating and drying of the article does not promote the effective recovery of the working fluid vapors by condensation on the cooling coils provided within the chamber because the vapors are required to diffuse across a large boundary layer owing to the ineffectual nature of the forced convection scheme used in the chamber.
In addition, the working fluid, which condenses on the cooling coils of the drying chamber, is free to flow down onto the lower surfaces of the chamber. These surfaces are being heated by the recirculating hot air from the heaters and by direct communication with the heat from the intermediate chamber. Consequently, the condensate may be reevaporated thereby continually replenishing the vapor within the atmosphere of the drying chamber.
Owing to the somewhat ineffectual means for recovering the vapor contained within the atmosphere of the drying chamber, when the door is opened to remove the article, losses of the working fluid result through convective interchange of its vapor-laden atmosphere with that of the exterior environment.